function [gr_blk_den gr_blk_map gr_blk_vt] = utl_cal_grad_blk(blk_vt)
[blk_rows blk_cols num_blk] = size(blk_vt);
gr_blk_map = zeros(size(blk_vt));
gr_blk_vt = zeros(num_blk);
gr_blk_den = zeros(size(blk_vt));
for i = 1:num_blk
    iblk = blk_vt(:,:,i);
    [fx_blk fy_blk] = imgradient(iblk);
    mg_blk = sqrt(fx_blk.^2 + fy_blk.^2);    
    gr_blk_vt(i) = mean(mg_blk(:));
    gr_blk_map(:,:,i) = mg_blk;
    gr_blk_den(:,:,i) = gr_blk_vt(i);
end
end